This invention generally relates to an apparatus for measuring physical properties of fluids and more particularly to a simplified product sampling apparatus for making on-line measurements of flowing materials.
In production processes, it is often desirable to measure physical properties of products that flow through some sort of flow line as they are flowing. For example, in the production of wet corn milling, there is a need for an apparatus to provide moisture content measurement. Such a mechanism must be sufficiently robust to withstand a variety of harsh industrial environments. With an apparatus of this type, the corn milling industry, for example, could generate a substantial savings in industrial energy consumption because corn drying processes could be programmed to terminate at precisely the desired dryness level. Other examples of industries requiring flow line measurement systems having high reliability, low operating costs, and economical implementation are those producing food, lumber, textiles, agricultural products, tobacco, coal, and paper.
In broader applications, not necessarily directed to flow line measurements, various means for obtaining liquid samples have developed. Existing devices for liquid sampling fall in two broad categories. The first category includes devices that simply provide a means for extracting the desired product samples. The sensing or measuring devices that are to be used for product analysis are typically physically and mechanically separate from the on-line system. The other category of devices are used in systems having a sensor downstream of a point at which the sample is diverted from a flow line. Sensors for both categories of sampling systems could include a wide array of non-intrusive testing techniques, including means for measuring low resolution nuclear magnetic resonance, electrical resistivity, density, and dielectric constant, for example.
For example, U.S. Pat. No. 4,744,255 issued on May 17, 1988 to Jaeger typifies the first category of product sampling systems. That disclosure shows a product sampling apparatus for communicating with the interior of a vessel containing a liquid. The product sampling apparatus extracts a liquid sample of predetermined volume from the vessel and conveys the sample to a point of collection. The invention is directed to a device for collecting a number of samples over time and combining them to obtain a composite. A plunger having a product-containing recess is used to convey the samples to a larger collection tank, so that desired measurements can be made. A limitation of this type of device is that such devices do not permit "instantaneous" sample-and-measure techniques. Also, the samples are not returned to the flow line. If the product requires a special environment, the device would not maintain those conditions, so as to permit returning the sample to the flow line in the same condition.
Using a thief product stream to divert part of the main product stream to a sensor exemplifies the second category of product sampling apparatus. Although these devices are "on-line" in the sense that the product can be measured as it flows, a disadvantage of this approach is that the product in the thief stream may not have the same characteristics as the product in the product stream. Also, a thief product stream poses potential problems related to achieving a product stream flow through the comparatively smaller thief product stream pipe. By the time the product is diverted and measured, the measurements thus obtained may not be real-time representations of the product flow.
There is a need for an on-line sampling apparatus that permits real-time measurements. The device should be sufficiently rugged for an industrial production process environment, have low-maintenance and operating costs and capable of being readily interfaced to the control processes of an industrial plant.